A lot of development has been made in semiconductor elements made of gallium nitride compound semiconductors, i.e., group-III-V nitride semiconductors (hereafter referred to as nitride semiconductors). Some of the applications of nitride semiconductors are: blue LEDs used as the light sources for illumination, back light or the like; LEDs used for multicoloration; and LDs. The manufacturing of nitride semiconductor in a form of bulk single crystal is difficult. Accordingly, GaN is grown on top of a substrate of different kinds, such as sapphire and SiC, by utilizing the MOCVD (metal organic chemical vapor deposition) method. The sapphire substrate is excellently stable in a high-temperature ammonia atmosphere in the epitaxial growth process, and is especially used as a growth substrate.
The manufacturing of nitride semiconductors by the MOCVD method is carried out, for example, in the following way. Gas of an organic metal compound is supplied, as the reaction gas, to the reaction chamber in which a sapphire substrate is installed as a growth substrate. The temperature for crystal growth is kept at a high temperature of a range approximately from 900° C. to 1100° C. The epitaxial layer of GaN semiconductor crystal is thus grown on top of the sapphire substrate.
However, the GaN semiconductor layer that is grown directly on top of the sapphire substrate by the MOCVD method has a hexagonal pyramid growth pattern or a hexagonal column growth pattern, so that the surface of the GaN semiconductor layer has a myriad of irregularities and has an extremely unfavorable surface morphology. Fabrication of light emitting elements is extremely difficult by use of a crystalline layer of a semiconductor that has an extremely unfavorable surface morphology with a myriad of irregularities formed in its surface, such as above-described one.
In a method used for the purpose of solving the above-described problem, the crystal growth of the nitride semiconductor is preceded by the growth of an AlN buffer layer on top of a growth substrate. Specifically, a low-temperature AlN buffer layer with a film thickness of a range from 100 to 500 Å (angstrom) is formed on top of the growth substrate at a low growth temperature ranging from 400° C. to 900° C. Since GaN is grown on top of the AlN layer that serves as the buffer layer, this method has an advantage of improving the crystallinity and the surface morphology of the GaN semiconductor layer.
According to the above-described method, however, the buffer layer has to be grown under strictly limited conditions. In addition, the film thickness of the buffer layer needs to be strictly set within a very narrow range from 100 to 500 Å. For these reasons, it is difficult to achieve a high yield and, at the same time, the improvement in the crystallinity and the surface morphology of the semiconductor. In short, the method is of little practical use.
Accordingly, a proposal has been made, as described in, for example, Patent Document 1 and Patent Document 2. The proposal is to replace the low-temperature AlN buffer layer with a low-temperature GaN buffer layer that is formed on top of a growth substrate at a low growth temperature ranging from 500° C. to 800° C., and then to grow the nitride semiconductor crystal on top of the low-temperature GaN buffer layer.    Patent Document 1: Japanese Patent No. 3478287    Patent Document 2: JP-B-8-8217